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Has there ever been an airliner design involving reducing generator load by installing solar panels?
Has there ever been a trial/project/design, in which solar panels were incorporated in the design of a commerical airliner (e.g. on the wings), with the aim to reduce fuel consumption by lowering the generator workload?
If no, why not?

I'm not aware of any. And I can see several important reasons why not: solar panels are heavy, they're fragile, they require a lot of maintenance, they require a lot of wiring.

@jwenting the actual solar panels aren't heavy at all. Most of the weight comes from structural reinforcements, which you don't need if you implement it into an existing structure such as the fuselage or wings.
Nevertheless I don't think it would be worth it.

Airliners are ridiculous energy annihilators(1). It's like every single passenger driving in their own car. It's like asking why Warren Buffett does not cut his empty toothpaste tube open to not waste the remaining toothpaste.-- (1) Of course the energy is not annihilated in the strict sense -- energy is always only transformed. But it is transformed into an unusable state (from dense chemical energy to dispersed heat) by increasing the entropy.

Solar energy is just a religion.  The amount of energy gatherable per unit area is ... totally trivial.  Indeed, the example of aircraft points out how utterly useless solar energy is.  Solar cells are a fantastically ingenious invention for, say, calculators.

As you include trail options, there have been at least two planes which have been powered completely by photovoltaic panels, Solar Impulse: https://en.wikipedia.org/wiki/Solar_Impulse it isn't commercial, isn't in anything like mass production, and is very slow, but they did do a round-the-world trip in the second one.

@Fattie Solar energy is perfectly workable on the ground, where getting 200W per square meter is fine.

The course of action that could be viable would be to use ground-mounted solar arrays to generate power, and then store it either as a synthetic fuel, or in batteries for use in flight.  Worth noting that the navy is working on the synthetic fuel idea, albeit not solar, as they operate airplanes off of floating nuclear power plants and would find it convenient not to have to replenish hydrocarbon fuel stores for the air wing and assorted accessories as often.

I hav heard that the wind is pretty strong up there. Why not use a wind turbine instead of solar panels?

@TonioElGringo I LOLed!

@Fattie The actual religion is believing that solar panels don't work. An area the size of Spain covered in solar panels would generate enough power for the entire world. Or 0,34 % of the  LAND AREA to solar panels. The windmill parks needed for that would be 10 times as big in area, and using windmills to power your laptop & phone requires way more infrastructure. PS Trumps recent rambling, completely incoherent rant against windmills ("wind sounds cause cancer")  is because of his ideology of GOLF COURSES over breathing and oxygen, and had nothing to do with cancer. Or TV.

@TonioElGringo a more serious answer would be that [A]. Windmills on planes is not a bad idea, it would recoup SOME of the spent energy. But present day cultural capitalism is against you. And the airflow IS TOO used for airconditioning, saving energy. Airplanes don't drag a fullblown AC system up there. But [B] The winddrag would slow down the plane (but not to the point it would cause the plane to lose lift capability. and [C]  A windturbine on a moving vehicle is almost a pull up by your bootstraps kind of paradox, if you ignore point [A]. Which you should not.

@GwenKillerby Windmills on planes is a bad idea because you're using fuel to spin a fan to move the plane to spin a windmill (that slows the plane down again) to spin a generator. Why not just connect the generator to the engine directly and skip all the wind stuff? And in fact, that is what they do. Excluding RATs for emergencies.

@immibis why not? because using wind to generate AC/cooling is skips the step of using extra machine power to power an AC system. Literal windmills on planes would be bad because it messes with the streamlined aerodynamics etc. There's nothing wrong with using clever air intakes, thats being done now already.

If we used windmills on the plane, it would be wise to additionally cover the windmill blades with solar panels.

If no, why not?

While I can't say categorically that it's not happened, I'm pretty sure.
Solar radiance is approx. 1kW/m^2. A 737 has approximately 100m^2 wing area. Solar cells are approximately 20% effective.
If you covered the entire wings in solar panels, that would work out to 20kW of electrical power at best. At night, it would be close to zero extra power.
Jet fuel contains ~43MJ/kg of energy. 20kW is 20kJ/s. For a 2 hour flight, the total energy produced would be 144MJ, or comparable to energy in 3-4 kilograms of jet fuel.
Turbines is not 100% efficient, so let's say that with all losses in engine, 25% of the power in the fuel is available as electricity. That means you'd need 12kg of fuel to provide the same amount of electricity as the solar panels.
12kg of fuel. That's probably far less than the solar cells will weigh, probably by a factor of at least ten. In addition, the you don't have to carry around already burnt fuel, unlike solar cells, which you will have to carry around.
Edit: I found another answer on this site, that claims extra fuel use is on the order of 0.125kg/kWh. I don't know if that's correct or not, nor do I really care. It doesn't change the conclusion, it only makes jet fuel even more favorable.
In short the amount of power provided by solar cells is tiny compared to the energy contained in jet fuel. And that doesn't even touch on the mechanical requirements of a wing...

Right, although a not completely unconceivable setup would be to have thin-film cells on the wings instead of paint. Solar cells can be made very light. The more fundamental problem is that there's just not really much area available, especially with reasonably high aspect ratio wings – and lower aspect ratio would mean higher drag, which would again defeat the point.

Black solar cell paint could also heat up the underlying wing structure. If aluminum, that's not good as heated fuel expands. If composites, that's not that good as heat is bad - that's why composite structure are painted white to help keep them cooler.

The problem with generating "only" 20kW by solar panels is not only that it is a tiny proportion of the total, but it's more electrical power than you actually need most of the time. For example the battery backup system on a 737-800, designed to provide 30 minutes emergency power if all other power generators fail, is rated at less than 2kW. Adding yet another system to convert the tiny amount of "excess" solar energy into mechanical thrust would make the concept even less practical.

@leftaroundabout the quality of the paint job is actually critical for the aerodynamics of the plane. Any "roughness" on the wing surface from the installation of solar cells (e.g. at the joints between sections of panel) would potentially mess up the boundary layer behaviour and reduce the stall margin.

Also, having solar panels at different angles to the sunlight introduces complexity. And an airfoil, by definition, is a curved surface.

Sure, there's lot's of small reasons why it's difficult. But those could probably be solved if it was economically viable. But it's not. It's technically difficult, and there damn close to zero reason for it.

There is this though: https://interestingengineering.com/boeings-solar-autonomous-aircraft-can-fly-forever-and-its-due-in-2019 and https://en.wikipedia.org/wiki/Solar_Impulse#Solar_Impulse_2_(HB-SIB) The latter being already operational, the first being technically more relevant.

@alephzero I imagine that this power would be used mainly for non-critical systems, like passenger entertainment, WiFi, and power outlets.

@paul23 that's not an commercial jetliner. As a proof of concept it's been done. For some applications, such as surveillance drones, it may make sense.

That's probably far less than the solar cells will weigh, probably by a factor of at least ten try another order of magnitude.  20kg/m^2 is not uncommon, and even light solar panels work out to about 10kg/m^2 so figure on maybe 1000kg for such an installation - just panels.  Now add the inverters and auxiliary equipment to feed it into the existing power grid, plus the extra fuel to carry that extra load.  No way this is coming out energy positive.

Or... to put it another way, you would burn much more fuel just carrying the extra weight of the solar generation equipment than said equipment could displace in fuel required for electrical generation.

@J can you show an actual  calculation for that, or are you just guessing?

@GwenKillerby A 737 is about 25% fuel when loaded.  Add 1000kg of payload, add 250kg fuel (~310L).  Back of the envelope.

@Joey Please don't edit to put units into that fractional format. Nobody writes units like that and, IMO, it makes the post harder to read by making the text bounce up and down.

@DavidRicherby: Then reject the edit. I don't have edit privileges here. But at least leave the non-breaking spaces before the units and the ², which do make the units easier to read (along with being just sensible typography).

@Joey I did reject the edit but others voted to accept. I commented to advise you for the future. I agree that the non-breaking spaces are good but it takes so long to edit out all the <sup> and <sub> tags that whoever decided to roll back the edit (which is just one click) presumably didn't feel it was worth their time.

No, there are several reasons:

Fragility v Efficiency v Weight:  the most efficient solar panels are rigid and heavy, which is bad for a wing structure. Flexible and light panels do exist, but they are half the efficiency. They also have limitations to how much flexing they really can take, the constant flexing of a wing, vibrations, cycles between hot and extreme cold at altitude all make it a punishing environment for that kind of technology. Covering the fuselage would mean less flexing, but then you'd only have a few panels positioned right at any one time to create electricity
Weight: In addition to the weight of the panels themselves you have all the other technology to make them work, like regulators, power conditioners, power storage, delivery wiring
Complexity: This is yet another system to maintain, and it would be complicated to do so. If a panel breaks you'd have to take apart the wing to get at it
Cost: you'd need solar panels that are efficient, flexible, durable and light. That all adds up to expensive panels, far more than is worth it
Limited window of use: Obviously solar panels are no good at night, but they are also only generate electricity when they are oriented at least partly towards the sun. If you're going to cover the wing then the sun must be a good 30-40° up before you'll get appreciable power from them

So it's a lot of weight and cost for a technology that isn't going to generate power for much of the time the airplane is in use.

This answer is not really correct.  (1) the actual answer is that solar cells provide what can only be described as "no" power, within rounding error. (2) the difficulties mentioned (cost, engineering difficulty etc) would, indeed, be instantly overcome if solar energy was 10,000x more powerful than it is (indeed everything on an aircraft is very expensive, difficult to make).

I don't follow you @Fattie. Solar cells do provide power, it's why they're being installed on homes all over the place....

They provide a minuscul amount of power, in terms of the question asked though.  Consider vid's answer.  At the absolute theoretical max it would provide the equivalent of "a few KG" of jet fuel.  (!)  That's why I said it is zero within rounding error.

There's more than one aspect to this @Fattie, and there was no point in repeating vidarlo's answer.

"Flexible and light panels do exist, but they are half the efficiency" - are you sure? The absolute thinnest and lightest "thin film" photovoltaics may be less than half the efficiency of crystalline cells (~10.5% vs ~26.7%), but panels using multicrystalline cells (~22.3%) are cheap and easily available and are as light and flexible as a laminated lunch menu. I use a folding panel like this with around 22% efficiency to charge my laptop when I'm away from power sources.

@Fattie At the risk of sounding risque, perhaps coarse, (but it is outmatched by your senseless remarks) ............... if you REALLY think solar panels provide no power would you be willing to hook your private parts up to a solar panel and ...... switch it on? It would just tingle, so it's not a problem, right? That would be just a case of putting your money (or rather..... johnson)  where your mouth is, no pun intended.

@GwenKillerby, I appreciate the humour (ouch ! :)  ), when I said "no power !" I was partially being hyperbolic, but do note that quoting the actual sentence - the immediately following words - [in an ariliner milieu] solar cells provide "no" power, within rounding error.   Please note that that sentence is literally precise and accurate in an engineering/basic math sense.  If you simply entirely covered a 787 with hypothetical 100% solar panels, the total power produced would be zero within rounding error compared to the two jet engines.

Not purely on topic but there is a solar airplane. Solar supported airliner isn't out of the realm of possibilities, just solar tech isn't there yet. Also it would have to be economically feasible to even be considered.
Here's an article from 2016 about a solar airplane that traveled the globe.
https://www.theguardian.com/environment/2016/jul/26/solar-impulse-plane-makes-history-completing-round-the-world-trip

Solar Impulse 2 is basically covered in solar panels of probably at least 20% efficiency.  This gives it enough power to fly with just a single pilot, at about 60 mph during the day, or 30 mph on batteries at night.  Even with 100% efficient panels, that's still nowhere near the power to carry a cabin full of people at much higher speeds.  Modern high-speed air travel uses a ridiculous amount of energy; that's why it creates so much greenhouse gas emissions.  It would be nice if solar flight were viable for airliners, but it doesn't look like it ever will be.

"just solar tech isn't there yet" - that's not the problem. Even if solar panels were 100% efficient, they (and all the wiring, the motors they drive, etc.) would still weight much more than the tiny amount of fuel holding the same amount of energy. Solar panels make sense on the ground, where you have a lot of area and weight is not a big problem. Even if technology advances by ludicrous amounts to make solar powered airliners possible, the panels won't be on the plane, the solar power will be generated on the ground, and stored in batteries or fuel cells to be loaded unto the aircraft.

It is on topic enough! And still, if a plane with a wingspan of a 747 can carry one person, then a plane with a double that wingspan can carry 2 persons. The thing is how to combine wingspan with lift. Is there a limit to wingspan?  One possible solution is  unfoldable wings or a kind of sail that doesn't drag. In 100 years or less oil will be gone, and I  guess people will still wanna fly.

@GwenKillerby : No, that's not how it works. Oil will never be completely gone. The oil reserves of the planet are not like the fuel in your car, that you drive and then at one point the car suddenly stops because fuel just ran out. As we use up the more economically extractable oil, it will become progressively more and more expensive. When it gets expensive enough, we will use less and less of it. There are many alternatives to oil, it's just that oil is so much cheaper. Once it gets expensive enough, it will be gradually replaced, first where it is easier to do so.

For air travel oil is much more difficult (with current technology nearly impossible) to replace, so oil will be probably used for longer, and it will last longer because other industries will use less of it. And after that, either some currently inconceivable technology will be discovered, or we will continue to use oils extracted by other means, maybe from crops.

FYI, if you start a post with "Not purely on topic" it should probably be a comment not an answer. I get that you don't have the reputation to make a comment though.

@vsz Yours might be the most Trumpian comment I ever read on stack exchange.
To use your own claims against you: If oil will never be completely gone...... then why would it need to be replaced????  Riddle me that. Stuff is finite.
And oil reserves will OF COURSE be gone. Even coal, if we were stupid enough to use it will only last 400 years. Your dangerous fossil fuel religion is the one ideology that will genocide us. Starting with the poor, of course.

@GwenKillerby I just think you are not aware of the scale of numbers involved.  A big airliner produces on the order of 300,000 horsepower.  You can look up or calculate yourself how big an area of solar collector one would need to generate that much power.

@FATTIE y I just think you are not aware of the third or fourth answer, which clearly says that there is already a solar panel powered plane.
And NOWHERE do i say that you can fly a 747 on solar panels alone. Please read better. Also, learn to think better. You are denying that solar panels produce electricity. That is ... INSANE, even in this era of Trump. That's some #Antivaxxer level of denying. In 10 years time, poor, 3rd world Morocco will get 20-33% of all its power from solar panels. While richer countries play stupid.

hi @GwenKillerby ! - need to get worked up.  (1) the SolarImpulse experiment is a fantastic example of how ....... absolutely tiny/slow a solar aircraft has to be!  (2) let's say the discussion is only about supplementing just the electric generators on an airliner, using solar power.  Again, it's just utterly, totally out of scale.  Solar Panels are incredibly clever technology for certain things, but utterly out of the question for airliners.

@GwenKillerby : Before throwing personal insults around first please learn to read comprehensively, instead of focusing only on two words out of context.  The main problem with oil is NOT that it will one day be just used up completely (and it won't, because as easy-to-access supplies will get used up, it will become more and more expensive, up to the point it will no longer be economical to extract). Actually, if there were more oil reserves, we would be in much deeper trouble, as it would get more expensive much later.

I once worked for a company that made electronics for commercial aircraft (flight deck printers, Ethernet switches, digital chart recorders).
In addition to what others have mentioned on this thread, you also have to account for the fact that if a product is manufactured for aircraft in the US, it must comply AS9100 and FAR, and whatever standard the EU is using nowadays.  This includes rigorous testing to ensure that, not only is the device safe, but also that the device will not interfere with any of the critical systems of the aircraft.

How is this related to the question? Solar power is a pretty simple source of electricity to deal with compared to everything else, especially compared to a generator.

One consideration that makes solar aircraft less feasible is that the figure of 1kw/m2 is for sunlight striking the solar panel square on - i.e. perpendicular to the panel. Unless you're flying in the tropics at noon, an aircraft's wings won't meet that. Their insolation (the amount of power from sunlight) drops with the cosine of the angle from vertical incidence.
Regarding doubling 747 wingspan for two seats - how many passengers will settle for a 45 MPH / 39 knot / 72 kph flight speed (i.e. New York to London in 77 hours if no headwind)?

but Jim - EVERYONE would get a window seat


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What reasons exist for not designing airplanes with breakaway wings for crash safety?
As most crashes are on takeoff and landing, and fire is the real killer, most often. Why not have breakaway wings? That's where the gas is. Let's assume a plane has to make a hard landing..upon touchdown/contact jettison the wings. They serve no purpose at that point and they definitely kill people. Breakaway parts/modules are employed in many arenas, so why not wings?

Fuel is carried in more places than just the wings for one, and think of all the equipment that would be required to jettison a wing and the fail safes involved in such a system.

I agree, But a) it's not complicated b) the majority of fuel is in the wings c) where talking about a crash scenario...Fail safes is the only valid argument so far and that is easily overcome. Obviously, individual wing designs would be a factor.

The wings also add a lot of stability to the structure as it crashes, they absorb energy, and keep things upright. It may be more feasible to have a high-volume fuel jettison system rather than jettison the entire wing. The wing serves a really useful purpose in a crash. Or, you could just parachute the cabin but also not really in the realm of reality at this point.

I understand the physics. They really don't unless they touch the ground. And that...is what we want to prevent. The initial contact with the ground (runway scenario) is the body of the aircraft, maybe nose or tail but that being said if at that point...that instant,  the wings detach...more people live. I understand your point but I disagree with the relevance of the wings once the plane is on the ground.

In all other situations, "breakaway" doesn't sound like a desirable adjective to apply to wings.

What happens if your wing jettison switch is accidentally activated at FL300?

I see more answers in the comments so I'll put my answer here too as not to sway from the community.  The wings are not part of the main structure usually and need the most work to make them the strongest part of the structure connection.  Making that connection disposable ('jettison the wings') would require a heavy system.  I would not see how such a system would add to safety and yet see many ways of how it would reduce in the quality of the airplane; the details would be too much for an answer here, I believe.

You seem to use assumptions as if they were obvious facts, e.g. "They serve no purpose at that point" (wings help prevent the fuselage to start rolling -- how to assess if this has a positive effect?). By doing this you encourage answers using the same kind of bias/shortcut. So at this stage you are asking for opinions and I'm VTC for this reason, but you can improve the question to remove biases. Ben's answer shows where they are.

Why the "primarily opinion-based" votes? There are lots of quite factual reasons why this isn't done.

Before I VtC, I would ask the OP: will you accept any answer that answers your question, as asked: "what reasons exist for..."? Or are you really so biased towards your belief that breakaway wings are the panacea you seem to believe they are that no answer will suffice?

I would also ask the OP to "define 'crash'". What scenarios, specifically, does he/she have in mind that breakaway wings would actually benefit and be controllable to an extent that the wings don't come off too early or too late, to be of any use, but only at just the correct moment as to supply the supposed benefits.

Removing the wings would just mean less metal between the soft humans and the hard ground.  What exactly is it that you're thinking the wings do in a crash?

My whole point, and argument for, was the situation where........a plane is crashing Now yes or no...are they better off without the wings that contain huge amounts of fuel or not? Simple question.

"It's not complicated."
As with most things in aerospace engineering, it is actually a lot more complicated than someone not familiar with aircraft design might think. There are a number of reasons why this isn't done:
Wing Spars
Wings have to be really strong. They literally bear (almost) the entire weight of the aircraft in normal 1 G flight and multiples of the weight of the aircraft when maneuvering. For example, in a 60 degree bank while maintaining altitude, the load on the wings is twice the weight of the aircraft. Of course, airliners don't normally do 60 degree banks, but they're designed to be able to withstand loads much higher than what they would normally experience on a routine flight.
In order to keep the wings from separating from the aircraft under such loads, they're designed with a single, very strong structure called a wing spar that runs most or all of the wing span. This is not one spar per wing, but rather a single spar that runs from one wingtip, through the fuselage, to the other wingtip. This spar is responsible for holding up the weight of the fuselage during flight and the weight of the wings (and, depending on where the gear is located, possibly also the fuselage) while on the ground. Using a single structure running the entire span for the spar is the safest and most efficient way to bear the needed loads, as it removes any connections that would otherwise be possible points of failure.
By design, the spar is really hard to break. Separating the wings from the aircraft would require breaking the spar. This would require enormous amounts of force and/or heat. The systems to supply that force and/or heat would be heavy and also, by design, rather dangerous. Neither of these is a good thing for an airplane.

Main spar of a de Havilland DH.60 Moth Source: Wikipedia
Landing Roll
It turns out that the wings (especially of an airliner) are actually more useful than you might think during the landing roll.
During a landing, the primary methods of slowing down are wheel brakes, engine reverse thrust, and aerodynamic drag. All three of these, at least to some extent, depend on the wings being attached to the aircraft. The engines, flaps, and spoilers are all usually mounted to the wings. The wing flaps and spoilers aid in providing large amounts of aerodynamic drag during the landing roll. The engines also require fuel flow and, as you've noted, that usually comes primarily from the wing. The wheel brakes might have some effect without the wings, but it would be greatly diminished. Braking action requires weight on the wheels. A lot of this weight comes from the weight of the wing structures and more comes from the downward aerodynamic forces provided by the spoilers.
Attitude Control
During the landing roll, the wings are still aiding quite a bit in controlling the attitude of the aircraft. In particular, the ailerons are still effective during the high-speed parts of the landing roll and are used during that time to keep the aircraft level in the roll axis. Additionally, a tall and very long tricycle, by itself, isn't a terribly stable vehicle. Especially at high speeds, it would be more prone to tipping over without the wings attached.
Landing Gear
Take a look at the main landing gear on these aircraft:

Boeing 777 Source: Wikipedia

Boeing 757 Source: Wikipedia

Boeing 737 Source: Wikipedia

Airbus A330 Source: Wikipedia
As you will notice in all of these pictures, the main landing gear are attached to the wings.
It's very common in aircraft design for the main landing gear to be attached to the wings. This makes sense as the wing root is structurally a good place from which to support the weight of the aircraft and also happens to provide a nice, wide wheel base to keep the aircraft more stable on the ground.
If the wing separates from the aircraft at touchdown, the main landing gear goes with the wings. As you can imagine, this creates lots of problems for a landing roll. In addition to the loss of wheel brakes, you are now skidding down the runway on the bottom of the fuselage instead of rolling down it on wheels. Hopefully I don't need to go into the reasons why this is not considered to be desirable.
Risk of Failure
Any system can fail. If one of these systems accidentally activated (either by accidental command from the flight deck or by failure of the system,) losing one's wings while in flight is a rather bad outcome. Given that flights needing wings are extremely common and flights crash landing in such a way that jettisoning the fuel tanks would be desirable are extremely uncommon, having such a system around is probably more likely to kill someone than save someone.
Burning Fuel
While one might think that jet fuel is easy to burn, it actually turns out that it isn't. In fact, it's rather difficult to ignite it. This is intentional in order to minimize the chance of the fuel igniting during an accident. Even in the rather violent crash of Asiana 214 and the large post-crash fire, the fuel never ignited (see first paragraph of page 37.) So, the risk being mitigated here might not actually be as large as one might think.
Note, however, that avgas (used commonly in light, piston-engine aircraft) is quite different from jet fuel. Avgas is similar to normal car gasoline and will burn rather easily.
Cost/Benefit Ratio
As previously mentioned, a system capable of quickly breaking the main wing spar would be heavy. It would be expensive to design to be safe. It would be expensive to build it. It would also be expensive to carry all of that extra weight around on millions upon millions of flights per year. All of this cost and added safety risk is not deemed to be worthwhile to provide a system that is rather unlikely to ever be useful.

Re Attitude Control, anyone ever landed in a good crosswind?

It needn't be heavy if it's explosive, like the frangible bolts used in spacecraft, where weight really matters.  But the other objections remain.

@CamilleGoudeseune Enough explosives to break a wing spar in two would be a lot more than frangible bolts, though. You could design the spar in pieces and use frangible bolts to hold it together, but, obvious safety concerns aside, you'd introduce major weak points in the spar and would then need to add yet more weight to strengthen those. While it's probably not going to be tons, it won't be trivial either, considering the millions upon millions upon millions of flights it will be carried around for per potential use case. Also, these charges would be right beside the fuel tanks.

I can't think of an accident scenario where this might help. If you're hitting the ground hard enough to crash but have survivors, you're looking at an accident like TK 1951, where not only was the wreckage all in pretty much the same area, but it didn't even catch on fire. Aviation fuel is not as volatile as movies make it seem.
Maybe you are thinking of something like AF 358 which overran the end of the runway and burst into flames? Despite that fire everyone survived, but what if they realised it was going to overrun and jettisoned the wings whilst still on the runway?
Well the biggest problem to me is how to slow the aircraft down without wings. At higher speeds, the wheel brakes are ineffective without the use of ground spoilers. Without the reverse thrust from the engines you are guaranteeing that the aircraft will be going faster when it leaves the runway, leading to a much higher chance of casualties.
No piece of engineering is perfect either - what happens when it fails and the wings accidentally detach when everything is going well? What if they are flying in severe turbulence and the sensors detect high G forces and confuses that for a hard landing? Or if it a manual jettison, what if the pilots accidentally pull it, or a suicidal pilot does it while the other is looking the other way?
There are many other problems too, such as how some aircraft are designed with the gear support in the wings, that some have large fuel tanks in the fuselage, and the extra weight and cost of getting such a system certified. Ultimately this suggestion falls into the same bucket as every other drastic safety idea - in trying to fix a problem that barely exists, it creates a lot more in its place.

You are right. Crashes are rare. Fixing a non problem makes no sense. However, there are dozens, maybe a hundred crashes where the plane gets down and bursts into fire due to fuel from the wings. I'm not suggesting it's a perfect solution, I'm stating that wings=fuel=fire=death .....wings break off, more people live. As to the mechanics, fixed wing aircraft come in many configurations, some could be modified some not. Future aircraft could adopt this idea.

@Thatchweave the answer highlights the quantities of problems in regards to the (almost null) benefit. Making the fuel tank more resistant (antideflagration materials, separations inside tanks,...) is quite more effective and rise fewer problems (almost only extra weight and extra pumps).

Best most concise answer. I understood this all along ..I wanted to see feedback.

Nitpick: the main reason the wheelbrakes are ineffective at high airspeeds without the use of ground spoilers is because the wings produce lift which takes the aircraft's weight off the landing gear and limits its ability to slow the aircraft.  Without wings, that particular issue would be no more (although many others would arise).

Just wanted to add that if wings were to be 'disengaged' when needed/emergency-case, the aeroplane-fuselage or a 'metal Tube' now will fall faster than a human would free fall, there will be nothing to slow it down (assuming pilot/autoPilot can still control wing-control-surfaces or whatever its called).
If the aeroplane is very close to ground or on ground and moving, discarding the wings now means throwing them away with undercarriage, the wings would most likely tumble and continue to move with the plane and may be even into it.
But if the aeroplane is on ground and not moving and then the wings are disconnected, means the Plane still has wings next to it, but now the fuselage is below it. The fuselage could roll and make it difficult to evacuate.
Also, lot of aviation fuel (30% or more) is in Central tank + ACT. i.e. in fuselage.

Why would it fall faster than a human would fall? It should actually be slower because of the air resistance working on a larger body. Neglecting air resistance two bodies will fall at exactly the same rate (9.8m/s/s or so depending where on the planet you are), be it a bowling ball or a feather. Terminal velocity happens when the upward force of the air resistance equals the downward force of gravity resulting in zero additional acceleration.

@RonBeyer Because aircraft fuselage is designed to have air resistance as small as possible. Unlike human body.

The early 310s with "tuna tanks" were designed with exactly this scenario in mind.  The idea was that the tanks would keep the fuel as far from the cabin as possible and shear off in a crash.  Cessna abandoned this idea some years later when they incorporated aux tanks into the wings (310L) and then later into the nacelles (310N).
I do not know if data exists to quantify the effectiveness of this design feature.

I suppose I was referring or intending to refer to commercial airliners. None the less, the concept doesn't seem that far fetched.

The tip tank placement addressed a problem which was not related to the biggest killer in twins - engine failure and asymmetric thrust loading at low speeds causing departure from controlled flight.

I don't think this actually answers the question. Although I'm not going to D/V it, also no U/V.

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text:
Is a degree from an overseas institution likely to be a barrier to employment with a major US airline?
I'm currently enrolled at a university in Jamaica. However, on my school break I often come to the US for flight training. I would like to know if a major US airline would employ me after completing my degree, given that it is an overseas degree?

It isn't clear what type of job you are after. Are you wanting a pilots position? Are you training for your commercial/ATP? What type of operation do you want to work for? Corporate? Airline? Cargo? What degree are you pursuing?

I think that this question is really in regards to the commonly cited requirement of a four year college degree that the "Major" airlines will look for in a candidate, regardless of the other requirements such as certifications and logged time. In other words, does an "overseas degree" count?

I just started my privite license training.and yes i am working towards becoming  atp pilot.I have an associate degree in industrial engineering.an currently pursuing a degree in finance.

I often hear airlines emphasize on 4 year degree.thou i have no interest in working in finance department..I'm just pursuing a degree becasue its a requirement of most major US airlines.i was just curious if after I clock my hours and have all my ratings.if major airlines would "look down"onmy degree giving that I didn't attend a local institution

It is unlikely that you would be "looked down on" if you are otherwise qualified, however it is certainly possible that an airline would prefer to have an individual with an aviation related degree.

Without a considerable amount of time as PIC of a turbine-powered aircraft, I expect your chances would be slight.
Once your other qualifications, logbook, and resume are competitive, I doubt that where you went to college will matter very much.

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text:
Is there any plan to create GPS approaches to water areas for seaplanes?
Is anyone working on differential GPS approaches for seaplanes? They are a largely ignored resource with great capabilities for certain remote places

Given that getting into the water often means descending below terrain, and that fog likes to stay on the water longer than on land, I doubt that there will be any kind of RNAV approaches published for water. There are very few towered water airports in the world (and yes there are a couple). There's just not a good way to make sure that nobody or nothing is in the way.

They already exist. See M57, for example.

Yes, such plans exist. The Aarhus Harbour Seaport (ICAO code: EKAC) is planning to implement an RNAV approach to allow seaplanes to land in conditions below normal VMC, however, the authorities have yet to approve it.

Thanks! Isn’t RNAV based on VOR technology? The other issue of course is the water state, the wrong wave frequency can be quite dangerous

@Guyfacey Well RNAV can actually be based on several different types of navigation, but when RNAV is used for approach (as opposed to en-route navigation) it is usually (including in this case) based on GNSS

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text:
Can a US registered motorglider be flown in Canadian airspace when piloted by a pilot with the appropriate FAA glider license?
Is a PPL-Glider recognized as ICAO approved? Can a US registered motorglider be flown in Canadian airspace (or land at an appropriate Canadian airport) when piloted by a pilot with the appropriate FAA glider license? (NOT an add on to a PPL SEL)

Unless your license says on its face that it doesn't meet ICAO requirements (as FAA student, sport and recreational licenses do), then when you are flying an aircraft registered in that same ICAO member state, that state's licensing rules apply and all other states must accept it as valid regardless of their own rules.
So, in your example, if the FAA says your aircraft is a "glider" and you have a private glider license, then you're legal in Canada even if, for instance, Transport Canada thinks your aircraft is an "airplane" and would require an airplane license for their own pilots to fly it.

Also if it has a radio, you would need a radio station license, this isn't required in the US, but it is in Canada, even if flying an N registered aircraft.

@RonBeyer Agreed, but I assume there's plenty of other Q/A on that issue, so I felt it best to limit the response to what was actually asked.

Good to go without a medical?

@pericynthion I'm not 100% sure on that. New question?


text:
Does the stability of a mass change if the center of gravity is above or below the point of thrust?
I was wondering if it makes any difference if the mass is balanced above or below the point of thrust. See image below for clarification:

In the image you can see two configurations. The red arrow / point in both represents the force / location pushing against the mass (hull of craft). This force can be an impulse or prolonged force. the blue arrows in both represent the upwards force of the rotors. And the green arrow represents the gravity in both.
I would imagine that balancing (keep the craft hovering) would be easier with the mass below the rotors since then gravity helps pulling the center of mass back under the center of the rotors. While if the center of gravity would be above the rotors and it would be pushed to an offset gravity would help make this offset worse.
So does it makes any difference for the stability of the mass (center of gravity) is above or below the point of thrust?
just to be sure config A represents the mass and center of gravity being above the point of trust, and in config B it is below the point of trust.
I am have no engineering or physics background so please explain this as simple to me if possible :)
Let me know if this is not the right place to ask and maybe suggest me where to ask this then.

Please read about "pendulum fallacy" in answers to questions like this one. Or here.

The vid is way better than the ASE responses lol.  You should just post the video as an answer, or add it as an answer post to the linked answer.

The most stable way to maintain a hover is center of mass below and center of drag above.
What confuses many people is how a "pendulum" reacts while being accelerated, and how it reacts when air drag stops acceleration.
Placing the center of drag above the CG is commonly seen on aircraft tails.  This is one reason why "classic" aircraft tails are "upright". Wing dihedral serves the same purpose,
to roll a side slipping plane back to level.
Once hover stability is achieved, one may reduce drag above or add drag below to reduce stability enough to allow controlled movement.
Leaving the Center of Gravity underneath will increase stability where when the object is not accelerating.  But "pendulum stability" is more properly called "pendulum  and drag stability", as in a parachute.
So, in your case, keep the triangle above with light materials., but have your weight below the rotors, and just use the rotors to lift and propel the aircraft.  But if you want a separate (red arrow) source of propulsion, try to put the center of horizontal drag and CG in the same spot, and push there.

Aircraft tails are upright so they don't plow up the runway.

Well, then maybe they can plow fields that way then, great idea!


text:
Is it possible to fly using a small engine?
I'm currently a high school student trying to build an ultralight aircraft from scratch (I'm self taught and still learning from educational videos), but I don't have much savings (about RM 2.5k, which converts to roughly 587 USD), so I could only afford a small engine.
Is it possible to achieve a certain propeller's efficient rpm by using gears while still using a small engine? Like those gearboxes found in cars to increase rpm.

It's possible to fly without an engine.

How small? Wilbur and Orville Wright flew using a 12 or 13 hp engine. The Rutan Quickie could cruise at over 100 mph using an 18hp engine.

In case you don't already know them: EAA Malaysia Chapter 1090

Vehicle (car, etc) gearboxes don't increase RPM, they decrease it, trading RPM for Torque. Think about it, your driving down the road at 3000 RPM doing 60 MPH (96 KPH), your tires are only doing about 840 RPM. Engines don't produce that much torque so the gear box (and torque converter) multiply torque by dividing RPM.

@DanPichelman technically gliding isn't flying, but falling slowly

This would be out of your price range, but this guy's project to fly like Iron Man is interesting and you might find some of it's information useful

@James Shaver: It is quite possible to climb in a glider/sailplane or hang glider without using power.  The current record is something over 50,000 ft.  It's also possible to stay aloft as long as the pilot can stay awake.

The short answer is yes you can use some cheaply available engines to achieve flight however you are presumably asking about things like small car engines or motorcycle engines etc. The issue you have here is that these engines are inherently different from a use case standpoint than your typical aircraft engine. Generally speaking, most car/motorcycle engines rev too high to be used in an aircraft, you actually want to lower the RPM, not raise it. You can read up on it here and here and here. You can pull it off with a stepper gear box (like a turbo prop uses) and it was even been done on a piston plane back in the 80's by Mooney. The other problem you will have with car engines is cooling them, generally cars are watercooled, something that adds a lot of weight and is not ideal for flight. A two stroke or other similar air cooled motorcycle/scooter engine may be a better thing to use.
Many small engines will produce more than enough horsepower to achieve flight. Keep in mind the early piper cubs only had 40HP which can be easily found in many 2 stroke engines.
Your best bet may be to look for a used VW bug engine as they are very similar to aircraft engines and even were used in some air frames at one point in history. Due to the cars high production numbers the engines are fairly easy to come by.
You did not list a jurisdiction in your question but the regulations and legislation on this change from place to place, so you may need to see what exactly you are allowed to do in your jurisdiction in relation to home built and experimental aircraft.

I wonder if you can build a working aircraft for less than say an FAA fee/license would cost to actually fly it.

@BruceWayne I think it's $5 for the registration, and then they'll inspect it for free if that's required. At that price point, you might have to go dumpster diving for used duct tape to hold everything together even if you find the rest of your parts for free.

I would think the Cri-Cri would fall into this category even though it is a twin engine (2 x 15 hp).

OP states his money in RM so i guess he is in malaysia.

Gliders fly without an engine and they do well :) Where I come from, it is common (and legal) to use an old car engine and build it into your own UL plane. Consider getting a car that is very old (70's or 80's) - these can be usually find either for free (after a rear crash) or very very cheap.
These old engines are also easy to remove and fit into an aircraft. In Europe, old Trabant engines are very common for this, but I am sure each region has some car model that is old, unused and waiting in planty in old scrapeyards.
If your legislation allows building a non-certified motor into your plane, than this wold be the way to go. These engines can easily output 40 to 80 kWs of power which is way more than you need.
Safety notice: If you do this, the #1 rule is: Always remember the engine is non-certified and old, so always fly it like it should break the next minute - avoid flying over large forests and water areas.

I would not want an old car engine because they were made out of iron.  The newer ones are aluminum and have EFI (FADEC) and a lot more power/weight because tuned runners make a lot of power. (Tuned runners do not play nice with carburetor, but at-valve FI solves that). A place like south Asia should be awash in modern econobox engines like the Mazda A3.

"Gliders fly without an engine", sort of. Usually they need more than an engine to start flying, they need another aircraft. In the way you suggest, all aircraft can fly without an engine, gliders are just optimized for.

In order to minimize engine power, reduce speed. The power need of an aircraft scales with the third power of speed, and many designs of the 1920s successfully used motorcycle engines. A high aspect ratio and a low wing loading, like in a glider, will help to get airborne with just 25 HP. Successful designs with such small engines were the Daimler L 20 or the Messerschmitt M 17.

Messerschmitt M 17 (picture source)

...how on (or above...) Earth do you see where you're going in that one...?

@MichaelKjörling: Like in a taildragger on the ground. Look left and right and shift the head sideways so you can see straight ahead.

The answer is yes and no.  Can an airplane be designed to fly on very low power?  Yes.  The Wright 1903 flyer flew on a mere 12 hp gasoline engine.  The real question is not can you fly on a lower power engine but can a lower power engine provide a meaningful flight envelope while carrying a realistic useful load over a wide range of atmospheric conditions.
It all depends upon your performance requirements which you need to decide upon before engine selection, but the suggestion of using a VW Beetle engine is a good one.  Rotax's line of piston engines have also been a good choice, especially for LSA aircraft.

Actually, you'd be gearing the engine down, not up. Propellers become very inefficient if the tips go supersonic (as well as extremely noisy). You'd want to keep the prop at around 1800-2200 rpm, depending on the size of the prop. Smaller engines typically put out peak power at much higher rpm's than that, so they have to be geared down to match the prop to the engine's power curve.
A typical ultralight engine is the Rotax 582. Designed for ultralight aircraft use, it can sustain high power output for long periods of time. That is current production, so it can be a bit expensive.
Now out of production but they can be found on the used market fairly inexpensively, is the Rotax 503. It has been a very popular ultralight engine.
Motorcycle engines may have less than desirable reliability. Typically, they are designed for brief periods of acceleration, with long periods of cruising at low power output. If you take a stock small motorcycle engine and run it wide open for long periods of time, the cylinder head will probably overheat, blowing the gasket and leaving you with no power. When that happens, you are going to come down, regardless of whether there is a safe place to land, or not.
I have seen Rotax 503's with a gearbox for ultralight use, in the USD 1500 range. More than a small motorcycle engine, but cheaper than a funeral...

Keep in mind that I'm very interested in airplanes, but I don't fly anything full scale, so others may have better advice.
There's a very simple, very light homebuilt called the Sky Pup. I'm not sure, but I think the plans are still being sold. Many of these have been built, and they have a good reputation. I've read that the guys who designed it were aero engineers. The materials are mostly wood and extruded polystyrene foam. The instructions tell you that anything more than 20 hp is too much. You can find out more at machnone.com (My only connection with the Sky Pup is that I bought a set of plans and have read about it here and there.)  If you weigh less than 170 lbs, you might find this a good choice.
Reduction drives are common, especially with two stroke engines. Another approach is to use half of a VW engine, although some of the power will be wasted. These engines are much too heavy for a Sky Pup, though. There's a design called the Legal Eagle, though, which uses a 1/2 VW engine.
New paramotor engines are probably far beyond your budget, but they might give you an idea of what can be done. They all have belt drives to reduce the propeller rpm. However, keep in mind that reduction drives can have big problems, unless they're engineered just right or have been tested and modified many times.
Unless you are very good at scrounging, or prices are MUCH lower where you are than they are here in the USA, your budget won't go far enough. Maybe there are some glider designs, though, that could be done with less than $1000 (US).
Depending on how an "ultralight" is defined in your country, there may be a bunch of other designs that you can use. For instance, the Pietenpol Sky Scout, which is smaller than the famous Air Camper, is meant for 20 or 30 hp (not sure which). It wasn't known for climbing quickly, but I expect a modern, lighter engine would help with that. I mention it because it's an old, low tech design which may be cheaper to build.
There are MANY other choices out there. Whatever you choose, get some flight training before flying solo. And be really careful when you're building it.


text:
How does Pack Flow work on the A320?
I am referring to an Airbus A320. I have heard of Pack Flow but I don't understand it.
If you put it on High, does that mean that all the parts of the air conditioning system work faster and harder?
I have also heard that if you put it on High, that means the demand for air supply is high, and vice versa, i.e. if it's on low then the supply for air is low, but what about NORM, or Economy flow?

Source: FCOM A320 familly
From the flight crew operating manual (FCOM) about the pack flow selector (4):

Permits the selection of pack valve flow, according to the number of
  passengers and ambient conditions (smoke removal, hot or wet
  conditions).
   LO (80 %)  - NORM (100 %) - Hl (120 %).
Manual selection is irrelevant in single pack operation, or with APU
  bleed supply. ln
  these cases, Hl is automatically selected.
lf LO is selected, the
  pack flow can be automatically selected up to 100 % when the cooling
  demand cannot be satisfied.

So to answer your question directly:

The normal flow (100%) is when the selector is on position NORM.
Position HI adds 20% to the air flow, and LO removes 20%.

In addition:

In LO position, the flow can be increased by the controller up to 100% in case the temperature cannot be obtained with only 80%.
In case of single pack operation, the flow is automatically set by the controller to 120%.

"hat or wet conditions" - I presume that should be "hot or wet conditions". If not, could you clarify "hat conditions"? :)

@FreeMan: Hat off :-)

A tip o' the hat to ya'

Pack flow is air flow through the air conditioning packs. There are two packs, one operated by each engine. Normally the system works on two packs, with some portion of the cabin air filtered and recirculated. The entire system can operate one one pack, which then must set to HIGH.
Yes if you put the pack on HIGH flow it means that that pack and all its connected parts work harder.
There is a lot of information on how exactly the system works and how to set it to what, for instance here.


text:
When may passengers board the aircraft and who makes the decision?
Who decides when the passengers may board the aircraft? Is this a pilot or cabin crew decision, and what are the prerequisites / conditions (cabin checks completed, flight plan filed, ...?) for this decision?

It is a joint decision of the entire flight crew (flight crew consists of pilots and cabin crew).
It is mostly a decision by the most senior flight attendant because it is mostly a cabin issue. The prerequisite is, obviously, the cabin has to be cleaned and prepared. There are "cabin checks" to be carried out when the plane is empty. Typically, each flight attendant will be assigned an area. When everyone reports they're ready, then boarding can begin.
But there are factors from the cockpit that can delay the boarding process too. For example, ATC informs them that takeoff will be delayed for 2 hours. Or a maintenance is being carried out. So, the pilots would always been informed before boarding begins, but in most cases there'd be no objection.

Captain is always the person who makes this sorts of decisions. If the captain is absent, the successor of command decides.
There are some limitations, though.

a minimum number of cabin crew must be present (depends on aircraft type, roughly 1 per 50 passenger seats)
cabin checks completed
fueling not in progress
other company defined items completed (cleaning, catering etc)

For example in my company, pilots' presence is not always required.

Also, at many airlines, passengers may board or disembark while fueling is in progress.


text:
Which current commercial flights have the highest ETOPS ranking?
While the Airbus A350 is technically certified for ETOPS-370, what are currently the highest rated commercial ETOPS flights? Which airline actually operates flights under ETOPS-330 or ETOPS-370 rules today?

What do you mean by "highest rated", safety reliability, number of aircraft, distance?

@SteveKuo: with highest rated ETOPS flight I mean commercial flights operating under the highest (in minutes) ETOPS rules (330, 370, 420 minutes). That somehow translates to maximum distance from enroute alternate airports, but since ETOPS rules are in flight time and not distance, I prefer time.

Air New Zealand's Auckland (AKL) to Buenos Aires (EZE) is the longest ETOPS flight I know of, under ETOPS 330. It started in 2015:

Air New Zealand completed the first flight approved for 330-minute extended twin-engine operations (ETOPS), flying a 777-200ER powered by Rolls-Royce Trent 800 engines. The December 1 flight departed from Auckland, New Zealand, and landed in the Argentinian capital Buenos Aires about 12 hours later.

As for ETOPS 370, I'm not aware of eny regular commercial flights, but some candidates are Sydney-Johannesburg and Sydney-Santiago de Chile


text:
What were the historical safety statistics of airplanes?
There's lots of debates in the Space world about the safety margins of modern rockets (specifically the fact that the ITS isn't planned to have an abort mode as (in the words of many SpaceX supporters), it "should be as safe as airplanes").
Now, I know that modern airplanes are crazy safe (as in, the last fatal crash of a major US airline in the US was just a month after 9/11), but I presume that back in the 1920s seats on airplanes were sold commercially without passengers having an abort mode either, and those airplanes were much less reliable than modern ones.
How has the accident per flight statistics changed throughout aviation history?

Are you specifically asking about the 20s?

Your question is quite good but maybe a little too vague. You should precise what you mean by "fatal accident" (does it imply injuries? aircraft lost? does it include delibarated act to loose the aircraft --terrisism, suicide,...--?)

As it stands this question is very broad, maybe narrowing down to a specific decade might help?

I believe the OP is looking for a general statistics of accident rate of commercial air travel since ~1920s (perhaps one data point per 5 years / per decade), which makes sense and is not too broad.

Related: Airline safety: what happened between 1959 and 1962? which actually covers the period 1959 to today, e.g. in this graphic from Boeing. You seem to ask about the US only, the FAA was created in 1958, that will be difficult to get statistics before 58.

“isn't planned to have an abort mode as it ‘should be as safe as airplanes’” is a contradiction—airplanes are so safe exactly because they do have a contingency procedure for any problem that is not extremely unlikely (where extremely unlikely means something like less than once in a billion hours).

Accidents "per flight" is unfair when comparing a 10-passenger Ford Trimotor
to an 853-passenger A380.  Passenger miles is fairer.
Even fairer is accidents per passenger, ignoring miles,
because takeoff and landing have always been more accident prone than cruise.
But even easier to measure historically is fatalities per passenger.
Commercial airline travel as we recognize it started in 1926.
http://www.planecrashinfo.com/1926/1926.htm
reports 36 fatalities worldwide that year.
1926 had about 6000 passengers in the USA.
Luft Hansa probably had more. Let's generously guess 20000 worldwide.
So the fatality ratio was about one in 550.
In 1930, 385,000 passengers were "carried on domestic and foreign airlines under the American flag".  In 1935, nearly, 1,000,000 (Berkeley Daily Gazette).  PlaneCrashInfo in these years veers away from airliners, though, and I couldn't find reliable global crash or fatality info for those years.
During World War II, and immediately preceding and following it, statistics about nonmilitary air travel are bound to be noisy.
From 1950 to 1970, per-year fatalities went from about 1,000 to 1,500
(with a dip in 1955).
Over that same interval, per-year passengers went from roughly 50,000,000 to 500,000,000.
So fatalities per passenger dropped from 1950 one in 50,000
to 1970 one in 333,000.
That second figure is corroborated by another source:
The "fatality risk per boarding" in 1968-77 was one in 350,000.
In 1978-87, one in 750,000.
In 1988-97, one in 1,300,000.
In 1998-2007, one in 2,700,000.
In 2008-2017, one in 7,900,000.
-- A. Barnett, "Aviation Safety: A Whole New World," Transportation Science 54(1), 2020.
https://doi.org/10.1287/trsc.2019.0937 paywalled;
summarized at http://www.sciencedaily.com/releases/2020/01/200124124510.htm

Actually, flying passengers for revenue started in 1910 with an enviable record of zero fatalities until 1914 when the aircraft were pressed into military service. Heavier-than-air airlines started from 1914 on with many more to follow.

DELAG is fascinating.  Let me ask directly: is wikipedia's assertion about the Deutschland's "heavy from loss of hydrogen caused by the rapid ascent" bogus?  Rapid ascent into cooler air would increase, not decrease, lift;  and it was still too low to trigger its overpressure valves.

I have a book that says LZ7 it climbed above its maximum altitude and vented some hydrogen. I think that for the press demonstration flight they filled the gas bags almost completely, and the journey so far did not exceed 600 m. The storm lifted it above 1050 m. With a failed engine not much dynamic lift was possible (so it went up uncontrollably) and a snow storm added so much snow (not just water) to the envelope that it went down. This, however, happened in slow motion as the ship got stuck in the treetops and passengers and crew evacuated with a rope ladder.


text:
Is a pilot allowed to sleep during a flight?
While in a flight, a pilot wants to sleep.  Given that the aircraft has an autopilot facility, are they allowed to sleep for a while (e.g., half an hour)?
What would happen if it's not allowed, but they still did it?  Would air traffic controllers know?

Regardless of how official regulations may read, sleeping in a cockpit is not good piloting.

Since you are asking about rules/laws, could you please specify which country you are interested in the answer for?  This likely varies from country to country.

Airline flight crews generally consist of at least 2 pilots. One pilot is designated as pilot flying, the other is the pilot not flying, observing instruments and flight parameters. If required, pilots can use Flight Crew in-Seat Rest:

Flight Crew in-Seat Rest
  This is the process whereby pilots may take short periods of sleep (naps), while temporarily relieved of operational duties in accordance with carefully prescribed ‘controlled rest’ procedures, when part of a two-man operating crew of an in-flight aeroplane.

This is not communicated to ATC and only allowed with two pilots present on the flight-deck. It is prohibited to have both pilots participate in in-Seat Rest at the same time.
See also these related questions:How do pilots rest on long-haul flights?
What if pilot feels too tired before his flight?

It should be noted that this answer is not true for all countries. Specifically, it is my understanding that this is illegal in the U.S., though I believe it is legal in the E.U.

@reirab Even more funny then, since the original concept is from the NASA, as far as I know... .P

Yes, NASA did some research on the possibility of doing this, but the FAA ultimately decided to continue not allowing the practice and instead requiring the crews have sufficient rest before and after flights (and separate rest areas where they actually need to sleep on long-haul flights with 3+ pilots.)

The correct answer is that the answer to this question, like many in aviation regulations, varies greatly by country. As SentryRaven's answer mentions, some countries allow Flight Crew In-seat Rest. However, this is not a universally-allowed practice. In particular, sleeping in the flight deck is never allowed in the U.S. One (but not both) crew sleeping on the flight deck is allowed in Europe, though.
What hasn't seemed to have been addressed in the other answers yet is what I would expect (or at least hope) is the much more common situation of pilots sleeping on board: augmented flight crews. An "augmented flight crew" is when more pilots are assigned to a flight than the minimum number required to operate the aircraft (which is generally 2 on today's airliners.) This is typically used on long-haul flights, where airlines can have 3 or 4 pilots on board. At least in the context of the U.S., two pilots must remain awake and in the flight deck, but the other 1 or 2 pilots may sleep. This way the crew can rotate flight deck duty and sleep time.
According to 14 CFR 91.1061, 3-pilot crews may operate flights with scheduled flight times up to 12 hours and 4-pilot crews may operate flights with scheduled flight times up to 16 hours. In the case of either a 3-person or 4-person crew, at least two of them must be qualified to fly as pilot-in-command on the aircraft, while the other(s) must be qualified to fly as second-in-command. This ensures that someone qualified to fly as pilot-in-command remains on flight deck duty at all times. In order to use an augmented flight crew, adequate sleeping facilities must be installed on the aircraft for the pilot(s) who are not in the flight deck.
The answers to this question give more detail about the crew rest quarters that are common on airliners used on long-haul flights. These exist both for flight crew and for cabin crew, though, from what I understand, these are generally separate from each other.

Just to add that airplanes for long range flights are actually design to allow this to happen and there is an small room where pilots not in service are allowed to relax and sleep.

@TrebiaProject., Yes, this question addresses that more fully, along with pictures.

No the pilot is not allowed to sleep on the job. Not unless another qualified human relieves him of his duties as pilot.
An emergency can happen at any time in flight and the pilot must be ready to deal with it.
A pilot sleeping would result in him not responding to air traffic control but that can have other causes like a failed radio.
Airline pilots have strict work hours they need to keep to ensure they are well rested before starting a flight. If he feels he is too tired he is obligated to refuse flying and let a better rested pilot take over.

This answer is not correct. In many jurisdictions one pilot of a two pilot crew is allowed to "sleep on the job".

In most cases, only if he is not on-duty.
Most transoceanic flights have flight times exceeding the amount of time a pilot is allowed by most aviation regulators to be at the yoke in one stretch. In the U.S., the FAA mandates a maximum of 8 hours in one continuous block, which can be extended by a few hours if the pilot is then allowed to rest for a longer-than-normal time afterward. Qantas Flights 7 and 8 between Dallas and Sydney are in the air for sixteen and a half hours in one continuous flight.
In these situations, the aircraft has multiple flight crews, and one crew will relieve the other in mid-flight. While not on duty, pilots may do almost anything the confines of the plane allow, including retiring to a special pilots' quarters, usually tucked in an out-of-the-way place on the plane, not obvious to passengers. These quarters, while not exactly palatial, are still of a level that even first class passengers would envy:

(source: core77.com)
The cabin crew also have an hours limit that is exceeded by most long-hauls, and they also have their own space to get away from their jobs, though it's slightly more spartan:

While on-duty, the FAA regs used to allow for "in-seat rest", allowing the "pilot not flying" to rest his eyes for a while if nothing interesting was going on. However, an incident with Northwest Airlines Flight 188 in 2009 was the final nail in the coffin for any allowance for sleeping in the cockpit. ATC lost all communication with this flight for almost two hours while it was heading to Minneapolis. By the time contact was re-established, the plane was 150 miles past the airport and still at cruising altitude. In the interim, F-16s were readied to intercept (but never launched) as the plane had been feared hijacked. While the pilots insist they were using laptops to go over airline schedules (a breach of Northwest's parent Delta's cockpit policies, not to mention basic flight fundamentals), other theories suggest they simply fell asleep. Unfortunately, the CVR only had the last 30 minutes of the flight in its data buffer, while the critical time to determine what they'd been doing before radio contact was re-established would have been at least an hour prior to landing.
British Airways and Qantas as well as a few other airlines do allow pilots on duty to sleep in the cockpit, as long as one crew member is awake and at the controls. However, they cannot do so while flying in U.S.-controlled airspace.

There was also a British flight a couple of years ago in which both pilots did fall asleep, which kind of underscores the danger in allowing in-seat crew rest.


text:
What good reference books for aviation (ATC & AOC) systems are there?
I am looking for a good reference book or research paper that describes the services and applications offered by air traffic control and Aeronautical Operational Control in detail.
Everything I have read so far just outlines briefly what these services are intended for. I need something very thorough with all the standardizations and details that are being used in current aviation systems, particularly from a communications point of view. Any suggestions?
What are the best textbooks or reference books that aviation researchers or experts use?

I don't know exactly what you're asking for (it seems like a very broad question) but you could start with the FAA's ATC orders. Reading through that will give you a good picture of what ATC actually does. If you're specifically interested in standard radio phraseology, Appendix A of the orders gives the standard US phrasing for most things, with a lot of ICAO phrasing too. If that doesn't help, please try to make your question more specific.

This is extremely helpful. Thank you. Is there an FAA AOP order or a similar document on AOP?

I'd start with the Global Operational Data Link Document published by ICAO.
It covers pretty much everything data link.  What it doesn't cover, you'll find in the referenced documents.


text:
Are there any supersonic aircraft with strictly mechanical controls?
Are there or is it possible at all to create a supersonic aircraft with strictly mechanical (human powered without boosters) controls?
I know there was an attempt: Bede BD-10 but it never reached supersonic speed.
Have there been successful planes/designs that implemented it?

Interesting question! Welcome to av.se!

@mins yes I mean no amplification like pneumatic, electric or hydraulic - just sheer muscle power.

There are two kind of hydraulics, with and without power assistance. For instance, hydraulic bicycle brake merely transmit human power, just like a lever and without power boost.

There's one I can think of immediately, and that's the granddaddy of them all, the X-1. The X-1's control system was unassisted and it had no hydraulic system. The only part of the control system that was assisted was the horizontal stabilizer, which was operated using high-pressure nitrogen gas.
I think most of the early supersonic test airplanes were mechanically operated .

Yes I thought of X-1 but it was a very marginal case...

You asked for a successful design @Artyom, I'd say breaking the sound barrier qualifies.


text:
Where can I get information about the FAA "restricted like" license to fly a N aircraft in Italy / Europe?
I have recently bought a N-REG C172 in Italy since 2008 and in fact I have been told by the Italian NAA that my EASA License will be valid limited for flights in Italy. I understand if I wish to fly in other EU countries I should apply for a FAA "Restricted like" License which I can obtain symply by giving a short english language check in recognized FAA offices in Italy. Any Idea about this "restricted" option? Whom should I ask to get more info?

Why would the FAA have offices in Italy?

Why would you not just have your plane registered in Italy?

The license they are talking about is an FAA license issues on the basis of your Italian one, provided that license is up to an FAA standard. See @Pondlife's answer for some links, and here's one on the AOPA website.
The FAA license would be restricted to the privileges of your Italian one. For example, if you do not have privileges to fly at night on your Italian license then you would not be able to fly at night. If your Italian license defines the day as being shorter than the FAA you'd have to use the Italian definition.
To get one you have to show up in person at an FSDO unless the Italian FAA equivalent has an agreement in place where they can validate identity, etc.

I don't know what an FAA "restricted" license is, but I guess it may be a foreign-based license issued under 14 CFR 61.75; see this answer for more information.
It's an FAA private license that's linked to your Italian one and it gives you all the privileges of a regular FAA private license, including operating N-reg aircraft anywhere in the world. But note that if your Italian license has more restrictions than the FAA one (e.g. no night flying) then you still have to obey those restrictions.
The process to get one is described here but I believe you have to physically visit an FAA office in the USA to get it.

In the meantime that's exactly what I've been told by an ENAC (NAA Italy) inspector.

I'm just back from FSDO in Hempstead near NYC with my temporary license and would like to give a feedback to anyone interested.

follow instruction @ https://www.faa.gov/licenses_certificates/airmen_certification/foreign_license_verification/
obtain verification letter from FAA
Schedule a meeting with your FSDO inspector
follow instruction given by FSDO to log in application @ IACRA website  https://iacra.faa.gov/IACRA/Default.aspx
when visiting take your IACRA credentials with you + logbook + license and medical certification
During the meeting at FAA-FSDO you are required to complete your application in front of the FAA inspector and electronically sign (that's why you need your credentials) and prove your Logbook registrered hours as well show original Medical certificate and License, FAA will aslo verify your English knowledge ,in my case a nice conversation about general subjects with the two FAA gentlemen I had in front of me
You will finally be issued a Temporary permit valid 120days
Your definitive "restricted" FAA License will be delivered to your home address within 90days
Your license will give you  a permit to fly an N aircraft only where your original License is considered valid (Europe) and according to ratings and limitations of your original license

I suggest you to double check the information that "EASA License will be valid limited for flights in Italy". Is that specifically mandated by the FAA? Here in Italy is absolutely commonplace to have D and G registered aircrafts and fly them, also internationally, with EASA or Italian licenses.

D- (Germany) and G- (UK) registered aircraft are from EASA member states. N- (USA) registered aircraft are not only subject to EASA regulations when they fly in Europe, but also to FAA regulations. As far as I know, the FAA only allows operations of an N- registered aircraft on an FAA license or on a license of the country the aircraft is being operated in. So if you fly an N-registered aircraft in Italy, you have to have either an FAA license or an Italian license. If you then fly into France, you have to have either an FAA license or a French license. See the problem?

Thankyou for the clarification. Has the problem of multiple states sharing the very same licensing body been clarified by the FAA? From this point of view we are becoming the "united states of europe", it soon will not really make sense anymore to talk of italian or french license.

@Caterpillaraoz that might be a decent question to ask.

@Caterpillaraoz Yes it has, see this question and read the comments: voretaq7 asked the FAA for an interpretation and they confirmed it.

@Pondlife Thanks!


text:
Can I model propellers used in electric aircraft the same way as I model RC quadcopter drones?
I am writing an article about mathematically modelling a flying car, using D'Alembert's principle. I need some information about the propellers ... When I worked with the mathematical model of a small RC quadcopter I found that many researchers consider the thrust of the propeller proportional with the square of its angular velocity and the same for the torque of the propeller. I am interested in something similar for the bigger propellers ... Is the assumption still holding? Can someone give some numerical values for the constants? The flying car (VTOL vehicle) I am modelling has six propellers, therefore I need less then 1000 N of thrust for each propeller. Therefore my first question is: Is it ok to say

$F = a \cdot \omega^2$
$T = b \cdot \omega^2$ for big propellers? $F$ is the thrust of the propeller along its axis, and $T$ is the torques developed by the motor to rotate the propeller. Please give some examples of commercial products ... if possible. Thank you!

Why the down vote
?

The thrust T of a rotor can be given as:
 $$ T = C_T \cdot \rho \cdot A \cdot (\omega \cdot R)^2$$
With:

$\rho$ = air density [1.225kg/m$^3$] at sea level
$A$ = area of the rotor disc [m$^2$]
$\omega$ = angular speed of the rotor [rad/s]
$C_T$ = thrust coefficient.

$C_T$ depends on the configuration of the rotor: blade pitch, number of blades, blade chord etc. It can range from 0 to 0.01, without knowing more about the config of your rotor we can take the average at 0.005 the value 0.001, see update below.
The torque Q follows a similar format:
$$ Q = C_Q \cdot \rho \cdot A \cdot (\omega \cdot R)^2 \cdot R $$
For a given $C_T$, we can look up $C_q$ in the following figure, valid for a propeller solidity of 0.1. For a $C_T$ of 0.005 we find a $C_Q$ (= $C_P$!) of 0.0004, for the updated $C_T$ of 0.001 the $C_Q$ = 0.00015  Source:

Update
The above $C_T$ is valid for helicopter rotors. For quadcopters with their much smaller rotors, induced power losses, tip losses etc are much higher. This document gives a detailed discussion on $C_T$ of small rotors, although a direct comparison cannot be made since they use a different definition of $C_T$. With this in mind, and using the definition of $C_T$ as above, a better choice for $C_T$ would be 0.001, and the corresponding $C_Q$ is 0.00015.

No worries mate.

To be honest I am interested in something between helicopter and small quadcopters ... I am interested in a flying car of mass $\approx 500 kg$ and having six propellers ... According to Fig. 11 page 20 in that document I think I will consider $C_T = 0.002$ and $C_P = 0.0002$ ... Is this OK ?

Yes that would bring the rotors in the same size category as the tail rotor of the Robinson R22 helicopter, R = 0.533m, chord = 0.1m, 2 blades. Not easy to cross-reference all design data but your assumptions seem to be on the safe side.

Great! Thank you sir for sharing your knowledge!


text:
Will the G1000 cut off transmission over 35 seconds?
I'm new on G1000 system, it says;

If the push-to-talk (PTT) Key becomes stuck, the COM transmitter stops
  transmitting after 35 seconds of continuous operation.

I'm wondering if we keep speaking on COM over 35 sec, will it stop transmitting? Or, if I keep PTT open without speaking, will it stop transmitting?

35 seconds is a lot of talking, especially in aviation...

I've done clearance readbacks, where I was instructed to read back all the route waypoints even though we were cleared as filed, that would've taken that much time.

If you have more than 35 seconds of talking to do, shouldn't you break that into multiple pieces? "Clearance, blah blah blah blah blah, lots of talking, blah blah blah, break." Wait a few seconds to see if anyone else wants to talk. "Blah blah blah blah blah, over."

I haven't tried it for real with PTT, but from what I know about G1000, it has generic protection against any stuck keys (including those on remote keypads/panels). When a key is detected to be pressed for longer than about 30-35 seconds, it is declared stuck and the respective CAS message appears on the PFD. It is then temporarily disabled; however, when another press is detected, it will work again. You can try it yourself with any of the bezel keys.
In this vein, it doesn't matter if you speak or not. If you keep PTT pressed for longer than 35 s, it will cut the transmission, but if you release and press it again, it will start transmitting again.
(Note that most normal keys in G1000 are activated on release (this is very common in software). But if a key is stuck or you hold it for long, its function will activate after about 0.6-0.8 s anyway. Some keys have 'press and hold' operation (e.g. holding frequency transfer for 2 s tunes in the emergency frequency 121.5), and it will also execute after the elapsed hold time. And only much later the key will be declared stuck.)

Thanks, appreciate. Perfect explanation.


text:
What was this vehicle doing?
On a trip a few weeks ago traveling out of Charlotte (CLT), I took this picture of a vehicle at the airport.  Our plane was fueled as we were about to board but due to various factors, boarding didn’t occur until almost five hours later.  As we were finally about to board, this vehicle arrived and connected a tube to a port in the vicinity of the fuel input port.  My question is this:  what was the vehicle doing and what circumstances necessitate the usage of it?
Edit: Although not immediately obvious without zooming in, the yellow pieces visible are the ends of a spool around which the hose is wound.  It is not one single large tank as it appears from a distance.

Maybe adding the airport can help in solving this question.

Was it a tube for liquid or was it possibly a cable for electricity?

@wbeard52 I’m not sure.  It only arrived about 10 minutes before we ended up departing and it left shortly after it arrived.

This looks like an Aircraft Fuel Servicing Cart, which transfers fuel from an underground network to the aircraft. There are two types of vehicles that transfer fuel at larger airports with these fuel networks:

Hydrant trucks:

(image source: Wikimedia)

Aircraft fuel servicing hydrant vehicles do not have tanks. These vehicles
  connect to a pressurized Airport Fueling System hydrant and transfer the
  fuel to the aircraft through a filter.

(ICAO - International and FAA Fuel Fire Safety)
Towed Carts:

(image source: aeroexpo.online)

Aircraft fuel servicing carts are equipped with facilities to transfer fuel between a
  Airport Fueling System hydrant and an aircraft and are normally parked at a fixed
  location near a gate.

(ICAO - International and FAA Fuel Fire Safety)

The cart in your picture seems to be the latter of the two (a different model though). You can see the hose that will be connected to the aircraft on the metal staircase. The other hose on the ground connects to the hydrant (hidden behind the aircraft in your image). Your picture shows an Airbus A321, which has the fuel coupling typically located under the right wing (number 11 in the following image):

(image source: Airbus A321 Aircraft Characteristics and Airport and Maintenance Planning Document)

11 − REFUEL/DEFUEL COUPLINGS (OPTIONAL−LH WING)

Your picture was taken at Charlotte (CLT), which according to this page is equipped with a Jet A Fuel Hydrant Facility. Airports without an underground fuel system will use tank trucks (self-contained fuel trucks) instead:

(image source: shell.com)

NOTE TO SELF (and others): Fuel hydrant = Fire hydrant, but they serve VERY different purposes!

Just out of curiosity, is the cart parked in the little white circle because of this?

@dalearn I was wondering the same thing, but could not find it in this FAA Advisory Circular or any other document I could find...

My guess would be that this is a bowser used for aircraft de-fueling.
Sometimes, very seldom though, aircraft need to be de-fueled to lower the weight for take-off. For example: if the aircraft is refueled for a flight to a destination and back (which is common for various reasons), but for some reason the payload for the first leg increases, the aircraft could be loaded over maximum take-off weight. In that case, a certain amount of fuel could be defueled to lower the weight.
The de-fueling would be done via the same fueling port as refueling, but a normal fuel truck wouldn't necessarily be used.
You can see the fuel hose from the cart connected to the scaffold. The fixed part of the hose in the scaffold is there to facilitate climbing up and attaching the hose to the fuel port under the wing.
Also, there are no other inlets or outlets in the wing near the fueling port.

Where would the fuel have gone?  The fact that it didn’t have any prominent tanks was what originally caught my attention.

@dalearn many modern airports don't use fuel bowsers for normal operations but a network of fuel lines running under the ramp that lead to tank farms at the edge of the field. Fueling equipment uses standardised connectors to hook up to those underground lines.

What makes you think this was used for de-fueling? 99.9% of the time this would be for re-fueling the aircraft.

@Bianfable at first I looked at this image with my cellphone, couldn't quite figure out what the yellow round thing is on the cart. If it's a tank, it's too small for refueling, only 1-2 tons. It could be a pump and a hose reel, but I don't see a hose connected to the ground well. Also when de-fueling, the fuel would not be directly mixed with the "clean" fuel in the ground tank before it was checked for water etc, so it would be sucked in to a small separate tank.

@Sami I see, that make sense :)

@dalearn when de-fueling is required, the amount is usually quite low. So if the yellow thing on the cart is a tank, it would be large enough

That looks like the ground heating/cooling rig.  It is a long flexible hose that connects to the plane (snf the other end connects to a heater or cooler.  This way the plane does not have to run the engine or the APU at the gate.
-Skip

For some reason, I doubt that this is the purpose.  The individual seen in the photo eventually climbed up the metal structure under the wing and plugged the tube into a port of some kind.  The cart itself was rather small with no obvious tanks.

The cart itself looks like a tank on wheels to me.  Unless you are referring to the scaffold looking one on the right side of circle.  But the yellow one looks like a tank.

@MichaelHall  If you look carefully the yellow pieces are ends to a spool for the hose.  From the side, no significant tanks were obvious

Gotcha, I couldn't zoom in enough to tell!

@dalearn your first comment would be good info to include in your OP. Plugging something in under the wing is critical info in answering this question with some authority.


text:
What's the greatest height at which a bird has collided with an aircraft?
70% of birds fly below 1,500ft, 25% of birds fly between 1,500ft and 15,000ft, and only 5% above 15,000ft. What is the greatest height at which a bird has hit an aircraft? What species was the bird?

One got ingested at 37000 feet  http://www.mnn.com/earth-matters/animals/photos/9-birds-that-set-records-for-their-amazing-flights/highest-flier

What kind of height, or altitude? True altitude, pressure altitude, height above ground? In the upper flight levels, extremes of condition could theoretically yield variation of thousands of feet between true and pressure altitude, for example.

What I knew (and this doesn't necessary contradict you, only helps to set more percentual boundaries) is that 90% of the birds are within 3000 ft AGL. That's why airliners try to hold the speed between 160 and 180 KIAS when taking off or landing if ATC report bird spotting or collision from previous traffic. :)

While not at flight levels, I have during the winter somewhat frequently seen eagles over central New York, at 13,000 to 15,000. However, I do not know anyone who has hit one at those altitudes.

On November, 29 1973 a bird strike occurred at an altitude of 11,300 m over Abidjan. The engine was shutdown during the collision, and was not restarted.
Altitude

The altitude is that recorded by the pilot shortly after the impact,
  which damaged one of the aircraft’s engines and caused it to be shut
  down. The plane landed safely at Abijan without further incident.

Source
Species
It was determined the bird was a vulture:

The remains of the vulture consisted of five complete and 15 partial
  feathers from the wings (secondaries, lesser, and underwing coverts),
  tail, neck, and breast. Sufficient details arc apparent in these
  feathers to allow their certain identification as G. rueppellii, using
  comparative material in the U.S. National Museum of Natural History.

Source
Rüppell's vulture is a specie weighting 7 to 9 kg and living 40 to 50 years:

Source.
Record
Wikipedia confirms this is the highest collision recorded in the history of aviation:

An aircraft over the Ivory Coast collided with a
  Rüppell's vulture at the altitude of 11,300 m (37,100 ft), the current
  record avian height.

Source
Why such altitude for a bird?
This is well above Everest height, why does this bird flies so high, where there are not so many preys, pressure is only 25% of the pressure at ground level and temperature is about -57°C?

The reason suggested is that thinner air allows faster flight without much
  effort and saves precious energy. Flying at such a great height,
  vultures face almost no physiological problems with regard to
  breathing; they have much more efficient breathing system than other
  living beings, including humans.
Their flow of blood enables oxygen to be extracted from air, with far
  greater efficiency than any other living being. If the flight is in
  cloudless condition, low temperature at great heights too does not
  bother them.

Source

Birds at high altitude

The previous record altitude for a bird-aircraft collision was of a
  Mallard (Anus platyrhynchos) at 21,000 ft (Manville, Wilson Bull.,
  75:92, 1963), based on feathers that I identified from the strike.
  That collision occurred between Battle Mountain and Elko, Nevada, on 9
  July 1962.
Other high-altitude records of birds include:
sightings of migrating geese at 29,000 ft, over the Himalayas (Griffin,
  Bird Migration, Natural History Press, Garden City, N.Y., 1964), and
soaring Bearded Vultures (Gypaetus barbatus) at over 24,000 ft.

Source
Vultures are a known threat to aviation, as are other birds flying high:

Images and data taken from Science Focus article
In 2010, in Scotland, a Rüppell's vulture participating to a display was caught in a wind and was lost. Aviation operators had to be notified, in case the vulture was still in the vicinity.

With a wingspan of 10ft, it could do a lot of damage to a large
  aircraft. But it's also half the size of some of our small training
  aircraft and it could take one of them, or even a helicopter, right
  out.

Source

Damaged compressor after a bird strike (not the involved aircraft):

Source.

Was this pressure altitude, or height above MSL?

@JonathanWalters -- I believe it was originally reported as a FL by the pilots

How did they identify the flying object that impacted the plane?

@Aron: By a few feathers in the engine (the engine was shutdown during the collision, and not restarted). See the second article at the bottom of the first page. What I like in this industry is the depth of the investigations.

What I'd like to know is: what on earth do vultures need to do at 37'000 feet?

I would sure like to know how high the thermal convection likely went based on meteorological conditions on the day of the collision with the vulture over Abidjan.  Or whether topography and wind conditions would have permitted mountain wave.

Highest I've ever heard of is 40,000 ft.
On May 19, 2014 Atlas Air flight GTI8665, a 767-300 freighter, reported a bird strike at FL400 over Indiana. They were unable to tell what type of bird it was but the pilots reported that there were guts on the windshield. The windshield was broken and the aircraft was diverted.

Pilot: Center Giant 8665 Heavy
Controller: Giant 8665 go
Pilot: Believe it or not we just had a bird strike up here [...] left
  windshield, we're okay
Controller: Giant 8665 do you need any assistance there then?
...
Another flight: What altitude was that bird strike?
Controller: That was Flight Level 4-0-0
...
Controller: If you had to guess was it just one bird there, did you
  see any other birds in the vicinity of that hit?
Pilot: To the best of my knowledge it was a single bird sir, we do
  have guts on the windshield and the outer paint is cracked...
Controller: ... Definitely an anomaly I think [...] 4-0-0
Pilot: Yeah 4-0-0 he may have been on oxygen I dunno
ATC Recording

Has anyone analyzed this case further?

@UnrecognizedFallingObject I dunno. I just remember hearing that recording. I think there's only one species of vulture that is even capable of flying that high

The wikipedia article on the Rüppell's vulture has an interesting footnote which leads a pdf of The Wilson Bulletin dated December 1974(!) which details the following other instances of high flying birds, including the one mentioned here:

Collision between a vulture and an aircraft at an altitude of 37,000
  feet.-On 29 November 1973, a Riippells ’ Griffon (Gyps rueppellii)
  collided with a commercial aircraft at 37,000 ft over Abijan, Ivory
  Coast, western Africa. The altitude is that recorded by the pilot
  shortly after the impact, which damaged one of the aircrafts ’ engines
  and caused it to be shut down. The plane landed safely at Abijan
  without further incident. The remains of the vulture consisted of five
  complete and 15 partial feathers from the 461  462 THE WILSON BULLETIN
  December 1974 Vol. 86, No. 4 wings (secondaries, lesser, and underwing
  coverts), tail, neck, and breast. Sufficient details arc apparent in
  these feathers to allow their certain identification as G. rueppellii,
  using comparative material in the U.S. National Museum of Natural
  History. The previous record altitude for a bird-aircraft collision
  was of a Mallard (Anus platyrhynchos) at 21,000 ft (Manville, Wilson
  Bull., 75:92, 1963), based on feathers that I identified from the
  strike. That collision occurred between Battle Mountain and Elko,
  Nevada, on 9 July 1962. Other high-altitude records of birds include
  sightings of migrating geese at 29,000 ft, over the Himalayas
  (Griffin, Bird Migration, Natural History Press, Garden City, N.Y.,
  1964), and soaring Bearded Vultures (Gypaetus barbatus) at over 24,000
  ft (Ah, Birds of Sikkim, Oxford University Press, London, 1962).-ROXIE
  C. LAYBOURNE, National Fish and Wildlife Laboratory, Bureau of Sport
  Fisheries and Wildlife, National Museum of Natural History,
  Washington, D.C. 20560. Accepted 7 June
  1974.

Actually, it does provide an answer.


text:
Are there locations/altitudes with minimum speed requirements?
For example, a slower aircraft operating on the North Atlantic Track System would have an operational impact on surrounding/overtaking aircraft as well as potentially increasing ATC workload to ensure separation.
So, are there any formally encoded minimum aircraft speed/performance requirements based on operational environment?
(Maximum speeds are covered in this question and the topic of why aircraft aren't designed to operate at slower airspeeds is covered in this question.)

I have never heard of any procedures or airways that require a minimum airspeed. Having a minimum climb gradient is quite common, typically on departure procedures to ensure terrain clearance.
It generally works out in such a way that aircraft with similar performance tend to cruise at the same levels. Most passenger jets cruising around 30,000-40,000 feet all happen to have a cruise speed of somewhere in the mach .75-.85 range. Slower turboprops often cruise at about 25,000 feet, and below that various GA planes. Granted, there can be quite a speed difference for those planes cruising at about 25,000 feet and below (for example, an ATR 72 has a cruising speed of 275 KTAS, a Dash 8 Q400 is about 370 KTAS - they both commonly cruise at 25,000 feet). However, these levels are typically less crowded, so it is not so much of a problem. For the really crowded, higher levels, most aircraft have very similar performance.

The way it works is you file your flight plan for the NAT with a proposed Mach #, but when you get your Oceanic Clearance they will assign Mach #s for each segment of the NAT and that's what you have to fly unless cleared for a different speed.  The adherence to assigned Mach #s are the only assurance of separation once out of radar coverage.
Perusing the NAT Operations Manual I couldn't find anything that gives a minimum Mach #, but if you were unable or didn't want to maintain the Mach # assigned, you would probably be given the option of flying down below RVSM airspace, or taking an off-NAT routing like the northerly "Blue Spruce" route (which airplanes use when they have to stop in Iceland for fuel because they don't have the range for the NATs).

That's not actually a minimum speed limit, though. It's just an ATC instruction, just like ATC will often instruct aircraft to reduce speed during vectors for approach.